1. Field of the Invention
The invention relates to a process for the manufacture of mixed esters from hydroxycarboxylic acids and partial glycerides of fatty acids. In particular, the invention relates to the manufacture of mixed esters from hydroxycarboxylic acids and partial glycerides of fatty acids with a high degree of hydrophilicity. The term fatty acid means an alkylcarboxylic acid with 8 to 18 carbon atoms, which may be saturated or unsaturated.* FNT *The term partial glycerides means mono- and/or diesters of glycerine.
2. Description of the Prior Art
The use of mixed esters from hydroxycarboxylic acids and partial glycerides of fatty acids for solving emulsifying and wetting problems in the pharmaceutical, cosmetic, food and animal food fields is well known. At the same time, the use of hydroxycarboxylic acids which are physiologically safe is both partly desirable and partly necessary. Such hydroxycarboxylic acids comprise the group of lactic acid, tartaric acid, citric acid and malic acid. Difficulties arise, however, in preparing such mixed esters with a high degree of hydrophilicity. One reason for this is that these so-called "food acids" possess poor solubility at times so that direct esterification reactions are possible only in exceptional cases.
The direct reaction of partial glycerides of fatty acids with citric acid is known from German Auslegeschrift 12 78 423. This process, however, has serious disadvantages and its use is therefore limited. For example, it is only possible to react, at most, 1 mole of citric acid with 1 mole of a fatty acid monoglyceride. If the molar proportion of citric acid is increased, a large portion of this acid does not participate in the reaction and precipitates from the reaction product.
If, on the other hand, attempts are made to force the reaction to proceed by increasing the temperature, the citric acid decomposes in an uncontrollable manner. As a result, the hydrophilicity of the reaction product falls off. Also, because of the by-products which are formed, the reaction products are no longer absolutely physiologically safe.
It is furthermore known that the citric acid may initially be converted with acetic anhydride into an internal acid anhydride with the simultaneous formation of the acetyl derivative, and that this anhydride may be esterified with the partial fatty esters. In so doing, however, one does not obtain the pure citric ester of the partial monoglycerides, but rather, products in which the hydroxyl group of the citric acid is acetylated. When these reaction products are stored or used, acetic acid is liberated and impairs the odor and taste of the food to which these reaction products have been added.
The object of German Pat. No. 24 55 989 is a process for the manufacture of citric acid esters of monoglycerides and/or diglycerides of fatty acids by reacting citric acid with monoglycerides and/or diglycerides of fatty acids at temperatures of 100.degree. to 140.degree. C. The process is characterized by the fact that the reaction is carried out in the presence of amounts of acetic acid such that a clear solution results and is maintained during the reaction.
With this process, it has become possible to manufacture citric acid esters of partial glycerides of fatty acids with a higher degree of conversion than in the German Auslegeschrift 12 78 423. However, in this process, it is also necessary to use acetic acid which, with respect to the desired products of the process, must be described as a foreign acid.